Magnets may be broadly categorized as temporary or permanent. Temporary (soft) magnets become magnetized or demagnetized as a direct result of the presence or absence of an externally applied magnetic field. Temporary magnets are used, for example, to generate electricity and convert electrical energy into mechanical energy in motors and actuators. Permanent (hard) magnets remain magnetized when they are removed from an external field. Permanent magnets are used in a wide variety of devices including motors, magnetically levitated trains, MRI instruments, and data storage media for computerized devices.
High-performance permanent magnets, such as Sm—Co (coercivity HC=10-20 kOe) and Nd—Fe—B (HC=9-17.5 kOe), are generally intermetallic alloys made from rare earth elements and transition metals, such as cobalt. However, the high cost of rare earth elements and cobalt makes the widespread use of high-performance magnets commercially impractical. Less expensive magnets are more commonly used, but these magnets generally have lower coercivity HC, i.e., their internal magnetization is more susceptible to alteration by nearby fields. For example, ferrites, which are predominantly iron oxides, are the cheapest and most popular magnets, but they have both low coercivities or coercive forces ranging from 1.6 to 3.4 kOe and low values of magnetization. Similarly, aluminum-nickel-cobalt (“Alnico”) alloys which contain large amounts of nickel, cobalt, and iron, and small amounts of aluminum, copper, and titanium, have coercivity in the range of 0.6 to 1.4 kOe, which makes exposure to significant demagnetizing fields undesirable.
More recently, Mn—Al—C alloys have been produced by mechanical alloying processes. D. C. Crew, P. G. McCormick and R. Street, Scripta Metall. Mater., 32(3), p. 315, (1995) and T. Saito, J. Appl. Phys., 93(10), p. 8686, (2003) have shown that adding small amounts of carbon (e.g., about 2 atomic % or less) to certain Mn—Al alloys stabilizes the metastable τ phase and improves magnetic properties and ductility. Crew et al. (1995) produced Mn70Al30 weight % and Mn70.7Al28.2C1.1 weight % alloys by consolidating ball milled powders, annealing at 1050° C. and then quenching, after which the materials were no longer nanocrystalline. The resulting alloys had grain sizes of about 300-500 nm and exhibited coercivities, HC, of 1.4 kOe and 3.4 kOe, respectively. Saito (2003), produced mechanically alloyed Mn70Al30 weight % and Mn70Al29.5C0.5 weight % alloys that had grain sizes of about 40-60 nm and coercivities of 250 Oe and 3.3 kOe, respectively. In this study, the low coercivities reflected the limited formation of the magnetic τ phase, which was determined to be 10% in Mn70Al30 and 40% in M70Al29.5C0.5. K. Kim, K. Sumiyama and K. Suzuki, J. Alloys Comp., 217, p. 48, (1995), produced MnAl alloys that were ball milled, but never annealed. The alloys displayed no hard magnetic properties, with a low HC of 130 Oe. These Mn—Al alloys are made from relatively inexpensive materials, but the low coercivities remain a problem.